The present invention relates to a power steering system of the kind in which an oil pump is driven by an electric motor for producing a steering assist force.
In a power steering system of the kind disclosed in Unexamined Japanese Patent Publication No. 2003-237601, in case a hydraulic circuit is locked due to, for example, stoppage of an oil pump, movement of a piston of a power cylinder in response to a steering operation of a driver causes working oil in one of oil pressure chambers of the power cylinder to be pressurized and returned to a reservoir. Further, a negative pressure is developed in the other of the oil pressure chambers due to expansion thereof, thus causing the other of the oil pressure chambers to draw working oil from the reservoir.